Flue gas treatment systems are utilized in connection with many industrial applications, such as the treatment for removal or neutralization of certain chemical species and particulates from the gaseous medium discharged from a fossil-fired generating system. A typical system processes the major portion of the dirty flue gas in parallel through a plurality of reactors, such as spray-drier absorbers, combines the outlet flow from the reactors, and subsequently passes the combined stream through additional ductwork to particulate removal apparatus such as a baghouse or an electrostatic precipitator. A minor portion of the flue gas, approximately 10%, bypasses the reactors and is mixed with the combined stream upstream of the baghouse in order to ensure a sufficiently high discharge temperature from the baghouse so as to avoid condensation of the cleaned gas in a downstream discharge stack. The streams discharged from the reactors flow within ducts and typically the direction of flow of at least some of the streams must be turned prior to entry into the baghouse.
The turning and mixing functions have been performed separately. For example, turning is often effected through T-shaped, L-shaped, or obliquely angled sections of the ductwork, and has also included the provision of L-shaped or curved deflectors of singular width, so called single element deflectors, positioned within the ductwork. The mixing function has been carried out through configurations such as a T-shaped interconnection of a duct conveying the bypass stream into another duct conveying the combined stream. This interconnection has also included an angle connection, for example, discharge of the bypass stream at an acute angle with respect to the combined stream. Entry of the bypass stream can occur upstream or downstream of the initial mixing and/or turning of the gaseous medium discharged from the reactors. Concentric ducts have also been utilized whereby the bypass stream flowing in an interior duct is discharged into the primary stream in the same direction as the flow of the primary stream.
While such systems have operated for their intended purposes, improvements can be made. For example, the extent of mixing of the bypass and major streams, which are at different temperatures, can be less than desired and result in stratification or other temperature profile distortions. This is particularly a concern where spatial limitations do not provide a sufficient distance downstream of the point of mixing prior to entry into the baghouse or precipitator to allow for complete mixing. Even where large transport lengths are available, it is known that although the systems operate in a turbulent regime, the widths of the ducting are so large that distinctive bands of turbulence occur which are not sufficiently violent across the entire cross section of the duct to allow for adequate mixing. Distortions in the temperature profile further complicate control of the overall flue gas cleaning system. Additionally, the various means utilized for turning the direction of flow inevitably induce undesirable pressure drops in the flue gas treatment system. And, injections of a hot flue gas containing corrosive species, such as sulphur dioxide, have been found to cause localized corrosion at the injection region.
It is thus desirable to provide improved systems for mixing of streams of gaseous mediums at different temperatures. It is also desirable to provide improved systems for turning of gaseous streams in flue gas or other gaseous medium conveying and treatment systems. Preferably such improvements will moderate pressure drops and provide the capability for substantial mixing, particularly within a confined area.